(1) Field of the Invention
The invention relates to an overrun protecting control system for hydrostatic-mechanical transmissions, that is suited for use in tracklaying vehicles such as bulldozers, and more particularly, to a technique for preventing the occurrence of overrun, for example, when the vehicle rolls down a hill.
(2) Description of the Prior Art
One of such control systems for hydrostatic-mechanical transmissions is disclosed in Japanese Patent Publication No. 62-31660 (1987). According to the system taught by this publication, the angle of a swash plate for controlling the discharge of a pump in the hydrostatic transmission unit is adjusted according to the difference between a target engine revolution speed calculated from a throttle position and an actual engine revolution speed so as to make the actual engine revolution speed close to the target engine revolution speed.